Theoretical Study Of Ionic-pair Structures Of Some Ionic Liquids And Their Interaction With Solvent

Ionic liquids,as a new class of material,represent one of the major challenges in the field of modern chemistry.Their many unique physical and chemical properties, such as negligible vapor pressure,low melting point,high thermal stability,powerful solvent capacity and recyclability,make them idea candidates for application in organic synthesis and separation process and the use of ionic liquids opens up a wide field for future investigations.The interest in this field has increased spectacularly in the last few years.The experimental and theoretical investigation for ionic liquids will be of important theoretical and practical values.Theoretical studies of ionic liquids by computational methods have been greatly facilitated by innovation of computer technology and development of the methods. Recently,B3LYP methods have been applied for ionic liquids and have achieved the considerable results.In this dissertation,we studied ionic liquids with density functional theory (DFT)calculations.Our purposes are to shed light on the structures' details of their ion-pairs and their mixtures with organic solutions and hence obtain a better interplay between theory and experiment,Our results provide detailed information on the structures and should be helpful for the development and application of ionic liquids.The valuable conclusions in this dissertation can be summarized as follows:1.The research history and current state on ionic liquids have been briefly reviewed.In recent years,their experimental applications of ionic liquids as solvents for synthesis and catalysis and as alternative media have been investigated extensively; relative little is theoretical studies about them.2.The stable geometries of pyridine,N-alkylpyridinium cations,and the ion-pairs of N-alkyl pyridinium cations with F^-,Cl^-,Br^-,NO₃^- and BF₄^-have been investigated by performing density functional theory calculations at the B3LYP/6-31+G(d)level.The calculated results indicate that the structure of the pyridine ring in N-alkylpyridinium cations is generally similar to that of the neutral pyridine molecule,i.e.the pyridine ring retains aromaticity,and the N-alkyl side chain has little effect on the structures of the pyridine ring.It is also found that the anions are inclined to move to the vicinities of the C(5)-H fragment or between C(2)-H and N-methyl above the pyridinium ring in ionic liquids.There are always multiple H-bondings between the anion and cation with partial charges transfer.Furthermore, the longer the N-alkyl side chain is,the weaker the H-bondings between the cation and anion would be.3.The interactions between the methanol molecule and several anions, imidazolium cation and the ion-pairs are investigated in detail.The anions with the methanol molecule form X^-…H-O dimmers and in the case of cation-methanol complexes,they form[emim]⁺…O model;The longer the N-alkyl side chain is,the weaker the interactions between the methanol molecule and cations;The difference of interaction energies of these complexes indicates that the acidity of H on imidazolium ring are stronger than that on N-alkyl side chain,furthermore,H that on C(2)has the strongest acidity;The ion-pair could form nine complexes with the methanol molecule,The interaction energies between anion and the methanol molecule is larger than that between cation and the methanol molecule;In the mixture of methanol and the 1-alkyl-3-methylimidazolium chloride ionic liquid,comparing with in pure ionic liquid,the interaction energy between[emim]⁺ and Cl^- decrease.The population of charge has been changed and red shift or blue shift could be found.Above all are helpful for the explanations of the physical and chemical properties for ionic liquids.In this dissertation,the valuable conclusion has provided reliable verification and has lead a theoretical guide line on further studying of ionic liquids,even for the development of the whole chemistry field.